NOTES. 465 



ence of two or three specimens in tHe same moUuso. When the young 

 animal, having just found its way into its dwelling, begins to eat, it will 

 catch at every organic object that is brought into the branchial cavity 

 by the current, and so hinder a later comer from establishing itself in 

 the same place. But an instance observed by K. Vogt is quite unin- 

 telligible without some such hypothesis as I have put forward in the 

 text. Among hundreds of specimens of a species of Labrus, of which 

 about 43 per cent, were attacked by a parasitic Crustacean, LepotpMlus 

 only two were foimd which had two parasites, one on each side ; all the 

 others had but one, sometimes on the right and sometimes on the left j 

 but the number of those that had settled on the right side was consider- 

 ably greater than those on the left, as 27 to 16 per cent. They were 

 always attached to the lateral line. What in this case can have hindered 

 the establishment of several parasites on the same fish ? As it seems to 

 me, the only thing that proves unfavourable to a second parasite is some 

 deterioration in the juices of the fish by the first. 



Note 123, page 372. This assertion that no mollusca but these of the 

 genus Onchidium have such dorsal eyes is based on the investigations, at 

 once of great extent and of extreme anatomical accuracy, conducted by 

 Bergh, of naked marine mollusca, and on my own researches, carried on 

 with a view to this particular, into the structure of other land and water 

 mollusca. Here and there, certainly, we find eye-like specks of pigment 

 on the back or sides of the body, as in Spluerodoris punetata anipapU- 

 lata among the naked mollusca, and Margamta in the Conchifera, but 

 all Bergh 's researches and my own, with all the most modern instru- 

 ments, show these to be merely concentrated spots of pigment with no 

 connection in any instance with a nerve, and exhibiting no trace of 

 the typical elements of a true eye. 



Nate 124, page 381. A case perfectly analogous to that of the 

 Onchidium described in the text occurs among fishes of the family of 

 Scopelidse. These are deep-sea forms, -to which indeed belong some 

 of those described by Giinther as having luminous organs ; at the side 

 of the body or on the belly they have a number — varying according 

 to the species — of large silvery spots of difEerent sizes, and which had 

 already been spoken of as eyes by Leuckart in 1865. Still, until quite 

 recently, the accuracy of this view had been doubted in spite of the 

 statement of that very skilful naturalist. Quite lately, however, an 

 exact description by Dr. Ussow (published in the Bulletin of Moscow) 

 of the structure of certain eye-like spots in some bony fishes leaves no 

 room for doubt, so far as I see, that Leuckart was perfectly right ; all 

 the attributes of true eyes are to be found in the genera ChoAnModrnt, 

 Astnmeithes, and Stomias. But according to Ussow other species have 

 organs in similar positions, which he designates as glands. I must con- 

 fess that his representation has not convinced me of the accuracy of 

 this interpretation, and I should venture to hazard an opinion on the 



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